Many offshore platforms are constructed as tower-like structures consisting of a welded steel space frame or "jacket" founded on or in the sea bottom and extending upwardly to an above-water deck which carries the desired drilling, producing, processing, and living facilities. These structures require strong foundations which can resist varying combinations of static and environmental loads. At most sites, pile foundations serve this purpose. Typically, after the platform jacket is positioned on the sea bottom, a pile, consisting of a hollow steel tube usually open at its bottom, is inserted through each jacket leg. Piles are normally installed by large pile hammers which are used to drive the individual piles to the desired penetration. Alternatively, it may be possible to install an open-ended pile by other means such as pushing or vibration. After the piles have been installed to the desired penetration, they are welded or grouted to the jacket.
Foundation piles must be designed to transfer all loads from the deck and jacket of the offshore structure to the underlying soil. These loads consist of tension or compression loads along the pile axis and lateral loads acting perpendicular to the pile axis. The pile penetration required to resist these loads is a function of the pile shaft friction capacity and the end-bearing capacity. The capacity of the pile foundation, including both pile shaft friction capacity and end-bearing capacity, should be such that the dead load (in water weight) of the complete offshore structure and its facilities can be carried by the pile foundation. Further, during earthquake loading or storm wave loading, the piles should carry the additional loads and there should be no reduction in the capacity of the piles, thereby insuring the safety of the structure.
As is well known to those skilled in the art, the maximum available end-bearing capacity (hereinafter referred to as "available capacity") for supporting a pile depends on the type of soil below the pile tip. For example, when a pile tip is driven into clay, the available capacity typically ranges from about 20,000 to about 30,000 pounds per square foot ("psf"). However, when a pile tip is driven into dense sand, the available capacity ranges from about 200,000 to about 300,000 psf. The actual end-bearing capacity of a pile will depend on the extent to which the available capacity is mobilized. If the available capacity is fully mobilized, then the actual end-bearing capacity of an open-ended pile will be substantially equivalent to that of a closed-end pile having the same diameter.
When an open-ended pile is driven into the sea bottom, a soil column will form within the pile and the available capacity of the particular soil strata at the pile tip may be partially or fully mobilized by plugging of the soil column within the pile. As suggested by Randolph (Randolph, M. F., "Capacity of Piles Driven into Dense Sand", presentation to the XI International Conference of Soil Mechanics and Foundation Engineering, San Francisco, 1985), for fully drained conditions it is likely that the available capacity will be fully mobilized when an open-ended pile is loaded if the soil column within the pile locks-up. As used herein, "fully drained" conditions exist when the pore water pressure within the soil column which is developed upon loading of the pile does not significantly increase above ambient pressure (i.e. hydrostatic pressure), or alternatively, dissipates quickly thereafter.
During loading, the existing soil column within an open-ended pile will plug, or "lock-up", when the inside skin friction at the pile wall becomes equal to the end-bearing capacity of the cross-sectional area of the pile. The basic mechanism for this "lock-up" effect is as follows: As the open-ended pile is loaded, shear load is transferred into the existing soil column and eventually to the bearing stratum at the pile tip. This shear load manifests itself as an increased vertical stress in the soil column. It is well known that vertical stress in a soil column will create an increased lateral stress on the column wall, or in this instance the pile wall. This lateral stress increases the inside skin friction, or shear capacity, at the pile wall due to simple Coulomb friction which in turn again increases the vertical stress in the soil column. This behavior propagates up the soil column and, in this manner, the inside skin friction at the pile wall increases at an exponential rate causing the soil column to plug or "lock-up". Thus, a locked-up pile will bear substantially the same load as a closed-end pile having the same diameter.
During non-static (transient) loading, such as earthquake or storm wave loading, the existing soil column within the pile may only "partially drain" and, accordingly, may not effectively plug, or lock-up, within the pile. Thus, the available capacity may not be fully mobilized. Partially drained conditions occur when the pore water pressure within the soil column which is developed upon loading increases significantly above ambient conditions and does not dissipate quickly thereafter.
Wave and earthquake loading are typically applied to the pile foundation in a matter of a few seconds or less. Because of this short loading period, the soil column may only partially drain and the increased vertical stress in the soil column caused by the shear load, will be carried largely or totally by the water within the pores of the soil column. As a result, the soil column tends to develop low vertical intergranular stress and thus low lateral stress on the pile wall, and the inside skin friction at the pile wall may not increase enough to fully mobilize the end-bearing capacity of the pile: The existing soil column within the pile may slip, rather than plug within the pile.
Under transient conditions such as those described above, the actual plugging behavior of the soil column is difficult to determine and typically requires sophisticated modeling approaches such as finite element analysis. Soil column plugging behavior is influenced by (a) the installation method, (b) the amount of platform gravity load carried by the soil column at the beginning of design loading, (c) the degree of arching in the soil column, (d) the stress-strain behavior of the soil column upon loading, and (e) the rate of loading of the soil column. All of these are difficult to determine, and the effects of each are not well known. In addition to the foregoing, soil column plugging behavior is also influenced by the permeability of the soil column. Very permeable soils will develop lower pore water pressures over a larger region of the soil column and are more prone to plug or lock-up. The tendency of a soil to develop low pore water pressure is also difficult to determine.
For the foregoing reasons, behavior of an existing soil column in a pile under partially drained or transient conditions is difficult to predict, and therefore the ability of the existing soil column within the pile to plug, or lock-up, and thereby fully mobilize the available capacity of the soil is also difficult to predict. As a result, open-ended piles may need their end-bearing capacity increased to insure that the pile foundation can support the complete offshore structure and its facilities, even during wave and earthquake loading. One conventional method used to insure that a pile is fully plugged is to drill out the soil column and set a grout plug. Another method is to place a grout plug on top of the soil column. Both of these methods are time consuming and particularly expensive offshore. The present invention is aimed at providing a practical and economical method for increasing the end-bearing capacity of open-ended piles.